This invention relates to an ionization chamber for calibrating radiation units and more specifically to a zero displacement ionization chamber.
Radiation therapy has become well known and its use widespread for treating cancer. In many instances, the radiation machines used in this treatment produce doses of x-rays or gamma rays. An example of a radiation machine used for this treatment is a Cobalt-60 teletherapy unit.
As in any radiation exposure to a patient, the dose given during any one particular treatment is critical. The criticality is more than just a problem of overexposure but in cancer patients, for example, the problem of underexposure is just as dangerous. Overexposure of radiation treatment will provide side-effects over and beyond the killing of cancer cells. Underexposure may provide only temporary relief of the cancer growth while leaving part of the cancerous tissue to later cause the patient problems. Therefore, accuracy of dosage is a critical parameter in this type of treatment.
Since dosage is critical, it has become common practice in radiation therapy to have periodic calibration of the teletherapy machines. This calibration is traditionally performed by using an ionization chamber. The use of the ionization chamber includes placing the chamber either in a water phantom or in air and directing the radiation machine at the ionization chamber to expose the chamber to gamma or x-rays. This exposure induces an electrical signal in the chamber which is indicative of the dosage received by the phantom. More specifically, the ionization chamber comprises two electrodes separated by an air gap with a high electrical potential difference between them. Exposure by x-ray or gamma rays causes ionization of air resulting in negative and positive ions that gravitate to the electrode having an opposite polarity. Thus, a current is induced which is proportional to the radiation received. This current is in turn monitored by an appropriate meter and can be related to the patient dose. This dose can then be compared to the actual setting of the radiation machine.
The calibration procedures may be performed in a water phantom which simulates a patient for purposes of detecting the dosage of radiation that would be received by a patient under actual therapy conditions. Water has been found to be an equivalent tissue substitute for this purpose, however, Cobalt-60 machines can be calibrated by measurements in air and related to the patient dose by calculation. The International Commission on Radiation Units and Measurements and other groups recommend that calibrations should be accomplished at 5 cm depth in a water phantom. A complete discussion of comparisons between water phantom and in-air calibrations may be found in an article by W. H. Grant, III et al entitled "Calibration in Water Versus Calibration in Air for Colbalt-60 Gamma Rays", Medical Physics, Vol. 4, No. 1, January/February 1977.
Research in the area of calibration using a water phantom has identified a calibration error when using state of the art ionization chambers for Colbalt-60 machines. In the overall calibration of the radiation machines there are many parameters that need be considered to accurately determine the radiation dosage supplied by that machine. It has been established that the calibration factors for therapy machines can be divided into chamber-dependent and chamber-independent components. A discussion of the relevance of the chamber-dependent factors may be found in an article by P. R. Almond et al., entitled "Ionization-Chamber-Dependent Factors for Calibration of Megavoltage X-Ray and Electron Beam Therapy Machines", International Symposium on National and International Standardization of Radiation Dosimetry, Atlanta, Georgia, December 1977. One such factor is the displacement factor which depends upon thickness of the chamber wall, chamber wall composition, size of the chamber and the center electrode. When using the ionization chamber in water, all of these factors influence the displacement factor. It has been found that a calibration error of up to 3% can occur if this factor is ignored.
In a typical treatment of a cancer patient, a normal dose might be 6,000 rads over a six-week treatment period. For proper treatment of the patient, it is often necessary to accurately establish the exact amount of radiation received during any one treatment so that subsequent or follow-up treatment may be analyzed. In many of the Colbalt-60 teletherapy machines, the radiation exposure rate approaches 100 rads per minute, and an error rate in the vicinity of 3% is not acceptable, as discussed by R. Golden et al. in Cancer, Vol. 29, 1972, page 1468.
Theoretically, when the ionization chamber is placed in water, the water is simulating the tissue of the patient and the purpose of the calibration is to effectively measure the dosage that would be received by a patient as stated above. However, since the radiation rays are attenuated at a different rate in water than through the ionization chamber with an air gap which has substantially zero attenuation, there cannot be a true reading of the actual dosage received by the water phantom. The difference between the absorption path lengths in the water phantom and ionization chamber has been labeled missing phantom material since the ionization chamber has in effect displaced some of the phantom material.